The present invention relates to broaching tools and methods of manufacturing the same.
Broaching is a machining technique commonly used to form radial retention slots in disks (e.g., compressor, fan and turbine disks) for aerospace applications, as well as in other applications. Broaching provides for slot formation with relatively high dimensional accuracy desired for aerospace applications, with tolerances in the range of 0.0254 mm (0.001 inch) or less. During the process, a disk workpiece is initially formed using cast and/or other suitable techniques. Broaching tools are then moved in a linear manner and repeatedly brought into contact with the disk workpiece to incrementally remove material from the workpiece to form a slot. In a typically application, a series of discrete and incremental broaching tools are used (e.g., from 10-30 discrete tools), from a “rougher” broach tool used to first define a rough slot opening to a “finisher” broach tool later used to refine a final slot shape to close tolerances.
Conventional prior art broaching tools were each made from a single piece of high speed steel (also called tool steel) with a grinding process. FIG. 1 is a perspective view of a prior art broaching tool 10 formed by grinding. Only a single tooth portion of the tool 10 is shown in FIG. 1, but as discussed above the tool 10 would include multiple teeth (e.g., 12-60 teeth per tool) integrally made from a single piece of high speed steel and arranged in a row for sequential contact with a workpiece. The tool 10 includes a front face 12 and an opposite rear face (not visible in FIG. 1), as well as a cutting surface 14. The cutting surface 14 extends between opposite faces of the tool 10, and adjoins the front face 12 at an interface 16 and adjoins the rear face at an interface 18. In the illustrated embodiment, the cutting surface 14 has a lobed configuration. More particularly, the illustrated cutting surface 14 is configured so as to define half of a fir tree shaped slot in a workpiece (not shown). The cutting surface defines so-called female lobes 14F, which are like relative low points, depressions or troughs, and so-called male lobes 14M, which are like relative high points, hills or peaks. An axis A is defined relative to the tool 10 that defines a direction of travel of the tool 10 relative to a workpiece during broaching operations. The front face 12 is the side of the tool 10 that approaches the workpiece first, meaning that the interface 16 is the portion of the tool 10 that generally contacts the workpiece first on each pass of the broaching tool 10.
Conventional broaching tools made by a grinding process, such as the tool 10, have numerous drawbacks and limitations. In order to fabricate a broaching tool by grinding, tool manufacturers have lead times of many weeks or months. Moreover, damage to any tooth of the tool may require scrapping of the entire tool. Because broaching tools can have a substantial cost (e.g., $40,000-50,000 U.S.), it is desirable to extend the life of the tool as much as possible. Broaching tools usually can be sharpened a limited number of times, usually a maximum of about four times before they must be discarded. It is often possible to broach no more than 4-8 workpieces per tool, which contributes to a relatively high cost of production for broached disks. Furthermore, broach tools made from high speed steel typically operate at up to about 305 cm/min. (10 feet/min). Moreover, high speed steel broaching tools work reasonably well on titanium workpieces, but not as well on workpieces made from nickel alloys (e.g., powdered nickel materials).
A particular problem encountered with prior art broaching tools relates to inadequate tool clearance. Inadequate tool clearance results in undesired rubbing of a broach tool against a workpiece that produces metallurgical phase changes in the workpiece due to pressure exerted by the broaching tool, which debits the life of the workpiece. A particularly undesirable scenario resulting from inadequate tool clearance is the formation of a white etch layer (WEL) in the workpiece from undesired tool rubbing. Inadequate clearance can result from contact between a portion of a tooth of the broaching tool and the workpiece after the tooth has removed desired material from the workpiece but before the tooth has been moved past the slot location in the workpiece.
In order to make prior art broaching tools by grinding, a grinding wheel is formed to a desired shape and then moved back and forth along a single dimension parallel with the axis A shown in FIG. 1. Clearance can be provided in the tool 10 though grinding, but only to a limited extent due to the limited movement capabilities of the grinding wheel. A blank from which the tool 10 is made can be angled by a grinding machine table 20 such that an offset is provided along the cutting surface 14 of the tool 10 between the interfaces 16 and 18 at an angle α. That offset provided by the angle α provides tool clearance, so that portions of the tool 10 rearward from the front face 12 and the interface 16 are spaced from the workpiece during broaching operations to reduce a risk of rubbing. However, as shown in FIG. 1, limited single-axis movement of the grinding wheel parallel to the axis A allows tool clearance to be provided in only certain areas of the cutting surface 14 identified by stippling. These stippled areas of the cutting surface 14 tend to be relatively horizontally oriented as shown in FIG. 1, and tend to be at central portions of each of the female and male lobes 14F and 14M. Regions of the cutting surface 14 between the stippled areas, which tend to be relatively vertically oriented as shown in FIG. 1, have little or no clearance, due to the limited single-axis of movement of the grinding wheel. Some cutting surfaces of broaching tools can have nearly “square waveform” shapes, which can make it impossible to provide clearance along the entire cutting surface due to almost purely vertical portions of the associated cutting surface.
It is therefore desired to provide an improved broaching tool.